How to disable vector pass calculating on unwanted frames?
To use vector information for frame 10, information from frames 9, 10 and 11 are used. This make sense, because to get a sense of speed, Blender needs to compare the positions of objects before and after the current frame with the position in the current frame. 

Lets say that you want to use vector info for frames A to B. Is it the case that you get weird results for frames A-1, A, B and B + 1 only? In that case I would simply manually re-render those frames with the vector checkbox ticked.

It might be possible to use a math node in the compositor to automate the 
influence of the vector pass in your node setup, taking 0 as the input to the vector pass node for the frames I mentioned and the actual vector values for the other frames